DC-to-DC converters have been widely used in various power supply circuits. In a multi-phase buck converter, for example, each phase typically switches a pair of Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) connected between an input voltage and ground GND to generate a phase output. To stabilize and balance the phase outputs of the converter, the output voltage of the converter and the phase currents generated by the phases are fed back to the control circuit of the converter to generate optimized signals to manipulate the MOSFETs. For the feedback of each phase current, a current sense apparatus is used to sense each phase current, for example in the circuit proposed in U.S. Pat. No. 6,246,220 to Isham et al., a current feedback resistor is used to generate a current sense signal fed back to the control circuit thereof. Since the control to each phase is derived from the current sense signal generated by the current sense apparatus for the respective phase, the accuracy of the current sense signal will directly influence the balance between the phases of the converter and the performance of the converter.
FIG. 1 shows a conventional current sense apparatus for a single phase of a DC-to-DC converter 10, which comprises a pair of MOSes 102 and 104 connected between an input voltage Vcc and ground GND, and a control circuit 106 to switch the MOSes 102 and 104 to generate an output current IO flowing through an inductor L and a current sense resistor RS to charge a capacitor C to generate an output voltage Vout. Upon the output current IO flowing through the current sense resistor RS, the voltage generated across the current sense resistor RS isVS=IO×RS,  [EQ-1]where RS represents the resistance of the parasitic resistor of the inductor L. To sense the output current IO, a transconductive amplifier 108 is connected across the current sense resistor RS by its inverting input and non-inverting input, and a resistor RX is connected between the current sense resistor RS and the inverting input of the transconductive amplifier 108 for the virtual ground between the inverting input and the non-inverting input of the transconductive amplifier 108, to generate the same voltage VS across the resistor RX. Therefore, the current sense signal between the output and the inverting input of the transconductive amplifier 108 is
                              I          X                =                              Vs                          R              X                                .                                    [                  EQ          ⁢                      -                    ⁢          2                ]            From the equations EQ-1 and EQ-2, it is obtained
                              I          X                =                                            Io              ×              Rs                                      R              X                                .                                    [                  EQ          ⁢                      -                    ⁢          3                ]            By sampling and holding the current sense signal IX by a sample and hold circuit 110, it is generated a control signal
                              Is          =                                    k              ×                              I                X                                      =                          k              ×                                                Io                  ×                  Rs                                                  R                  X                                                                    ,                            [                  EQ          ⁢                      -                    ⁢          4                ]            where k is a constant, and accordingly, the control circuit 106 regulates the output current IO by switching the MOSes 102 and 104.
However, after the converter 10 operates for a time period, the temperature of the current sense resistor RS increases, such that the resistance of the current sense resistor RS changes. From the equation EQ-3, the current sense signal IX is proportional to the resistance of the current sense resistor RS, and therefore the accuracy of the current sense signal IX is degraded, thereby causing the control circuit 106 operating improperly.
FIG. 2 shows another conventional current sense apparatus for a single phase of a DC-to-DC converter 20, in which the inverting input and the non-inverting input of the transconductive amplifier 108 are connected to the source and the drain of the low-side MOS 104, respectively. When the output current IO flows through the low-side MOS 104, due to the internal resistance Rds of the low-side MOS 104, the voltage generated across the low-side MOS 104 isVS=IO×Rds,  [EQ-5]and from the equations EQ-2 and EQ-5, the current sense signal is
                              I          X                =                                            Io              ×                              R                ds                                                    R              X                                .                                    [                  EQ          ⁢                      -                    ⁢          6                ]            By sampling and holding the current sense signal IX by the sample and hold circuit 110, the control signal is
                              Is          =                                    k              ×                              I                X                                      =                          k              ×                                                Io                  ×                                      R                    ds                                                                    R                  X                                                                    ,                            [                  EQ          ⁢                      -                    ⁢          7                ]            where k is a constant. The control circuit 106 regulates the output current IO by switching the MOSes 102 and 104 based on the control signal IS.
Likewise, after the converter 20 operates for a time period, the temperature of the low-side MOS 104 increases, such that the internal resistance Rds of the low-side MOS 104 changes, and the control circuit 106 will operate improperly.
Therefore, it is desired a temperature-independent current sense apparatus.